Estimation of average surface currents from ERS SAR images of oil-tank cleaning spills

Synthetic Aperture Radar (SAR) images from ERS-1/2 have been used to study current-induced perturbations of spill bands in the Kuroshio east of Taiwan, stemming from ballast water of several ships that had been dumped after the cleaning of tanks during the movement of the respective ship parallel to Taiwan’s coast. Under weak winds, the variable surface currents normal to a ship’s track were the main cause of the displacement of spills relative to a straight line. The currents were associated with the interaction of the Kuroshio with the island of Lutao and ocean bottom variations as well as with vortex structures of different scales, which were observed near the Kuroshio boundary. The average current velocity was determined by dividing the magnitude of the spill displacement by its residence time, which in turn was inferred from the distance of the spill from a ship and the supposed ship velocity. The current shear zones manifested themselves on the SAR images as narrow light lines. Sharp shifts in a spill were observed at its crossing lines. The estimates of current shear, based on the magnitude of the shifts, agree with published data. The width of a spill band observed on May 20

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, 1994, at different distances from a ship, was found from the sections normal to it. The change in width as a function of the residence time may be well approximated by a power function with an exponent of about 0.86.